The present invention relates to ion sources.
Ion sources are widely used in a broad range of instruments, such as ionization gauges, ion guns, mass spectrometer-based residual gas analyzers, gas chromatography mass spectrometry (GC-MS), liquid chromatography mass spectrometry (LC-MS) and more. Various ionization mechanisms are employed in these and other ion sources, including electron ionization (EI), chemical ionization, photo ionization, surface ionization, electrospray ionization, matrix-assisted laser desorption ionization, etc. Of these ionization methods and sources, EI is one of the most widely used ion source, due to its high sensitivity, ease of generating high electron emission currents, approximate uniform ionization yield to all compounds and atoms, rich structural information in the resulting EI mass spectra, and the available extensive 70 eV EI mass spectral libraries that enable automatic compound identification.
Several types of EI ion sources are used, depending on their application. Among these, the two most widely used EI ion sources are the Nier type and Brink type ion sources.
The Nier type EI ion source is based on a closed ion cage volume with an electron emitting filament positioned outside this volume so that the ionizing electrons enter the ion cage through a narrow slit and pass the ion cage only once. A major feature of the Nier EI ion source is that the sample inside the ion cage is isolated from the filament and its degree of degradation on the hot filament is thus limited. Furthermore, the filament power is relatively low; thus, the ion source temperature can be relatively uniform and better controlled. Nier type ion sources are widely used in GC-MS instrumentation and also in some forms of LC-MS, known as particle beam LC-MS.
The Brink type EI ion source has an open ion cage structure and a longer filament that is relatively close to the ion cage. As a result, the ionizing electron emission current is higher than in the Nier type EI ion source and the ionizing electrons may travel several times inside the ion cage for increased ionization yield. However, the resulting very high ion cage temperature (typically 400-500xc2x0 C.) and the highly exposed hot filament near the open ion cage preclude the use of Brink type EI ion sources in standard GC-MS and LC-MS systems. The Brink type EI ion source finds wide spread usage in ionization gauges and gas analysis mass spectrometer systems.
In recent years, a new type of GC-MS and LC-MS has been under development, based on sampling with supersonic molecular beams (SMB) and electron ionization of vibrationally cold molecules while they are in the SMB in their flight passage through a Brink type EI ion source. An important feature that emerges from the use of SMB is that, due to the directional motion of the SMB, the sample compounds are not in contact with the hot filament and ion source walls. When seeded SMB are used with a light carrier gas such as helium or hydrogen, even if a small portion of the sample compounds scatter from the filament heated ion source walls, these compounds lose their high (hyperthermal) directional kinetic energy and thus, even if ionized, the ions of thermal vacuum background compounds can be filtered out by the ion source lens system. This vacuum background filtration process is based on the difference between the ion energy of ions formed from vacuum background compounds and those that were formed from SMB species due to their added directional kinetic energy as hyperthermal neutral molecules.
The vacuum background filtration is usually performed by the introduction of an ion blocking potential along the trajectory of the analyzed ions, such as at the exit lens of the quadrupole mass spectrometer (or at another ion lens element), with a retarding voltage that is sufficient to retard the thermal vacuum background ions, but not ions of ionized SMB species. However, this filtration is inefficient with SMB species that do not have high kinetic energy, such as compounds with relatively low molecular weight, in pure SMB, or when the seeded carrier gas is relatively heavy. In these cases, the differences in ion energies between vacuum background ions and ions of beam species are too small in comparison with the apparent ion energy distribution function, and vacuum background filtration is confronted with excessive SMB ion beam intensity losses.
While the 70 eV EI mass spectra of organic compounds in SMB almost always exhibited a molecular ion peak, for certain experiments it was desirable to eliminate all the lower mass fragment ions and remain only with the molecular ion. The standard way to achieve this soft ionization is to lower the electron energy. However, the reduction of the electron energy resulted in a reduction of the ionization cross-section, combined with an annoying further observation of significant reduction in the electron emission current. The reduction of ionizing electron emission current resulted from increased space charge effect between the filament and ion cage, which had a lower electrical field between them upon the reduction of the electron energy. This reduction in ionizing electron emission current can be especially significant (a few orders of magnitude) if one wishes to obtain only a few electron volts of electron energy for negative ion formation through electron attachment, and complex and expensive electron guns or other electron sources must be used for this purpose.
While working with a fly through Brink type EI ion source with a GC-MS system based on sampling with SMB, it was observed that the process of background ion filtration was incomplete and that the observed ion energy distribution function was broader than expected. Furthermore, the optimal ion source lens system voltages significantly deviated from what was found with computer-based simulations of the ion trajectories. Since these observations could not be accounted for, they were ignored for over a decade.
It was finally realized that these adverse phenomena and the inability to explain them resulted from the inappropriate assumption that the central ion cage element was unaffected by external fields. These conclusions emerged from unexpected results of experiments aimed at testing the optimal filament position in a standard Brink-type ion source. A more careful computer simulation of the electric field distribution inside the ion cage later revealed that the field inside the ion cage was not zero, and that external fields of the filament and of the electron emission repeller penetrated inside the fine mesh of the ion cage wall. The penetration of the field of the filament was especially undesirable, since it was close to one side of the ion cage and thus created an asymmetry in the ion trajectories as the ions were attracted to the filament side that had lower electrical potentials.
Based on these observations and conclusions, a new and improved EI ion source was constructed, in which the assumption of close to zero internal ion cage electric field is better fulfilled. The additional, new key element in this ion source is a second external field-insulating cage between the internal ion cage and the electron-emitting filament and electron repeller. Despite the fact that the addition of a second cage was initially thought to lower sensitivity, due to ionizing electron current losses on the second cage, and the increased distance between the filament and the inner cage could lead to increased deviation of the ionizing electron trajectories from the center of the inner cage, it has been found that these prior concerns are invalid.
In accordance with the present invention, there is therefore provided an ion source, comprising an inlet port for introduction of a sample into the ion source; an outlet port through which an ion beam exits; means for ionizing said sample; an ion formation chamber confined by an ion cage, and at least one electrical shield for shielding said ion chamber from the penetration of electrical fields affecting the ions inside said chamber.